Powder coating is a relatively simple process that has some strong economic advantages in two broad application areas and some limitations in each of the areas. Powder coating involves the affixing of a powder, previously selected for its property enhancing feature and finely ground, onto the surface of a substrate to be coated as a layer of powder and then, if a continuous coating is desired, subjecting the powder coated substrate to heat which melts the powder layer, allows it to flow, and fuses it into a continuous coating. Powder coating is a dry process that has significant energy and labor cost reductions, high operating efficiencies, and environmental safety advantages over liquid coating processes because the powder coating process does not require a volatile carrier for the purposes of coverage and flow. Powder coating is used in two broad application areas, applying coatings onto large surfaces and applying powder onto particulate substrates. Different powder coating methods to adhere the powder to the desired surface are used in the different areas.
There is a large demand to apply decorative finishes and protective coatings onto metal surfaces. Large area powder coating is an economical and environmentally safe method of applying coatings such as these. Achieving adhesion of the powder onto the surface of the substrate is usually done using a fluidized bed, plastic flame-spraying, or electrostatic spraying.
In the fluidized bed method, the thermoplastic or thermoset powder is placed in a suitable container and fluidized. The part to be coated is heated and dipped into the bed of fluidized powder. Upon contacting the heated part, the powder melts and adheres. However, particles less than about 100 .mu.m generally do not fluidize well, parts are usually of metal so deformation will not occur with heating, the powder is generally limited to thermoplastic materials, the container must be full of fluidized powder, and the minimum coating thickness is typically over 250 .mu.m.
In plastic flame-spraying, the thermoplastic powder is transported through a combination air/propane flame held over a surface to be coated. The molten powder contacts the surface and adheres. With this method, small and intricately shaped parts are difficult to coat, coatings are limited to thermoplastic materials, and the melting of a thermoplastic powder in a flame can degrade some polymers and possibly cause the formation of hazardous gases.
In electrostatic spraying, a high voltage source is used to establish a stable corona field in a powder spray gun. Powder particles are dispersed in an air stream and passed through the corona discharge area where they become electrostatically charged. The charged cloud is then directed to the grounded substrate, to which each particle is drawn by the positive-negative electrical attraction. However, generally only metal substrates can be coated, powder is limited to thermoset materials, the maximum coating thickness is typically about 75 .mu.m due to insulation of the metal surface and the electrostatic repulsive forces as the powder layer grows, and intricately shaped surfaces are difficult to coat.
There is an increasing demand for surface modification by particulate coating to improve such characteristics as flowability, dispersibility, wettability, bulk density, color, and performances in electrical fields. There is also a desire to save on usage of high priced and/or rare materials by bonding these materials onto lower cost carriers, to create new composite particulate materials and to shorten production steps and cycle times. Small area powder coating is an economical and environmentally safe method of accomplishing these modifications. Applications where small area powder coating is done include, for example, toners, cosmetics, pharmaceuticals, dyes, paints, ink, ceramics, powdered metals, food flavors, fine chemicals, catalysts, electromaterials, and biochemical materials. Achieving adherence of the powder to the surface of the core particulate is usually done by a mechanical fusion method.
In mechanical fusion, powder and core particles are usually premixed in predetermined ratios. The mixture is then fed into a chamber that may be heated. Rapidly moving parts in the chamber collide with the powder and core particles, causing them to collide at such velocities that they fuse to each other. Multiple passes may be used to create a thicker coating layer on a core particle or multiple different layers. Limitations include, excessive machine wear if the core particles are abrasive, a tendency to fracture hard core particles or break fragile core particles, difficulty in coating intricately shaped small articles, and difficulty in coating continuous articles such as fiber tow.
Many of the limitations of the large area coating methods and the core particulate powder coating method are overcome by coating magnetically propelled particles onto solid substrates. The substrates can be simple or very complex shaped articles of plastic, metal, or any hard material. The powders can be plastic, metal, or inorganic material. The process involves placing a preweighed amount of powder, small particulate magnetic elements and a substrate to be coated in a confined space. The contents within the confined space are subjected to a magnetic field of sufficient intensity to cause the small magnetic elements to move and thereby cause the powder to impinge upon and to coat the exposed surface. However, the process is a batch process the types of substrates and powders that can be used are limited. Additionally, in many instances using a white powder coating with small particulate magnetic elements results in coatings that are discolored and/or blackened.